In sub-micron scale integrated circuits, CMP techniques are used to create the planarity required in multi-level interconnect structures. Specifically, to create a planar surface for depositing an interconnect layer, e.g. aluminum, tungsten, or copper, an interlayer dielectric (e.g., silicon dioxide) is planarized by a polishing process. This polishing process uses a polishing pad, usually polyurethane, under pressure in frictional contact with the wafer surface. The polishing pad carries an alkaline or acidic slurry with fine abrasive.
CMP in semiconductor processing removes the highest points from the surface of a wafer to polish the surface, as described for example in Leach, U.S. Pat. No. 5,607,341, issued Mar. 4, 1997. CMP operations are performed on unprocessed and partially processed wafers. A typical unprocessed wafer is crystalline silicon or another semiconductor material that is formed into a nearly circular flat wafer. A typical wafer, when ready for polishing, has a top layer of a dielectric material such as glass, silicon dioxide, or of a metal conformally overlying one or more patterned layers. These underlying patterned layers create local protrusions on the order of about 1 .mu.m in height on the surface of the wafer. Polishing smoothes the local features, so that ideally the surface of the wafer is flat or planarized over an area the size of a die (a potential semiconductor chip) formed on the wafer. Currently, polishing is sought that locally planarizes the wafer to a tolerance of about 0.3 .mu.m over the area of a die about 10 mm by 10 mm in size.
To maintain uniformity over the polished surface of the interlayer dielectric and to provide wafer-to-wafer reproducibility (global uniformity) of the polishing process, the polishing surface, typically a polyurethane pad, is required to be conditioned during use or between uses. Conditioning is necessary to maintain a uniform, textured or profiled surface on the polishing pad.
Polishing rate and uniformity depend in a complex fashion on a number of process variables at the wafer-pad interface, significantly contact pressure, relative velocity between the polishing pad and wafer surface, elastomeric properties including hardness (durometer) of the polishing pad, physical and chemical properties of the slurry, and rate of chemical reaction.
Traditionally, CMP is performed using a planetary CMP apparatus. FIG. 1 is a schematic plan view of a planetary CMP apparatus 100. As shown in FIG. 1, CMP apparatus 100 includes a polishing table or platen 103, rotating in a direction indicated by reference numeral 105. Onto platen 103 is mounted a polishing pad 104. A silicon wafer (not shown) is mounted onto a polishing head 101 and is pressed against the surface of polishing pad 104. Polishing head 101 rotates the silicon wafer in a direction 109, generally in the same direction 105 of rotating platen 103. Additionally, an oscillating arm 106 reciprocates polishing head 101 transversely along an arc indicated by reference numerals 108a and 108b. Correspondingly, a conditioning pad (not shown) is mounted onto a smaller platen 102 and is pressed against polishing pad 104. Platen 102 rotates in a direction indicated by reference numeral 110 and is reciprocated throughout the CMP process by an oscillating arm 111 along an arc indicated by reference numerals 107a and 107b. Slurry is sprayed or applied by other conventional methods onto the surface of polishing pad 104 throughout the CMP process by a slurry dispenser 113.
Process control is difficult to achieve in a traditional planetary CMP configuration of FIG. 1. Nonuniform removal rates are produced at the wafer-pad interface due to the locally variable and complex motion of the polishing pad relative to the wafer surface.
FIGS. 2a and 2b are side and front views, respectively, of a linear CMP apparatus 200. An example of such a linear polishing apparatus is disclosed in Anderson et al., "Modular Wafer Polishing Apparatus and Method," U.S. application Ser. No. 08/964,930, filed Nov. 5, 1997, the disclosure of which is incorporated herein by reference in its entirety and which is copending herewith and assigned to Aplex Inc., also the Assignee of the present application.
As shown in FIGS. 2a and 2b, linear CMP apparatus 200 includes a continuous polishing belt 201 configured to polish one or more vertically supported semiconductor wafers, such as a wafer 207. Wafer 207 is held vertically by a polishing head 205, which presses wafer 207 against a polishing pad 208 attached to vertically mounted polishing belt 201. Polishing belt 201 is kept in continuous motion at a selected polishing speed within a range of approximately 1-10 ft per second or 0.3-3 meters per second by rotating pulleys 202 and 203. A center support 206 provides an opposing pressure to hold wafer 207 at a preselected pressure within a range of approximately 1-10 PSI, or 6-70 kPa, against polishing pad 208. Polishing head 205 rotates in a predetermined direction indicated by reference numeral 216 and is reciprocated laterally by an oscillating mechanism (not shown) across the surface of polishing pad 208 along a path indicated by reference numerals 211a and 211b. Thus, the combined motions, of polishing belt 201, polishing head 205, and an oscillating mechanism cooperatively provide linear polishing of the surface of wafer 207.
While FIGS. 2a-2b show only one side of the polishing belt assembly being used for wafer polishing, polishing heads 205 can be positioned on both sides of the polishing belt assembly of CMP apparatus 200 relative to a plane of mirror symmetry containing the axes of both pulleys 202, 203, thereby effectively doubling the total wafer throughput. A slurry dispenser 213 is mounted proximate to polishing belt 201, to apply slurry to polishing pad 208. A linear pad conditioning assembly 204 is mounted proximate to polishing belt 201, to provide conditioning for polishing pad 208 attached to the surface of polishing belt 201. Linear pad conditioning assembly 204 includes a linear motion mechanism that causes a conditioning surface to travel in the directions indicated by reference numeral 209 transversely relative to the direction of belt travel, indicated by reference numeral 215. The combined motions of the linear motion mechanism and polishing belt 201 provide linear conditioning of polishing pad 208.
Similar to center support 206, a conditioner back support 217 typically provides an opposing pressure to hold conditioning assembly 204 at a preselected pressure within a range of e.g., 1-10 PSI, or 6-70 kPa, against polishing pad 208. When polishing heads 205 are provided on both sides of polishing belt assembly 200, a linear pad conditioning assembly 204 can be provided on each side of polishing belt 201.
In linear CMP processing, automatic control over the lateral position of the moving polishing belt is required. Without lateral position control, the belt can eventually slip laterally and slide off either end of a pulley.
Some attempts to control lateral belt position have required slow and unreliable human intervention. Tension sensing by means of strain gauges has been applied to related processes involving linear web transport, e.g. printing and paper or foil making (see for example Breen, "Enhancing Web Processes with Tension Transducer Systems," Sensors, August 1997, pp. 40-44). Conventional instrumentation has proved difficult because of the hostile environment, including rapidly moving and vibrating machinery, water spray, and airborne slurry and other particulates.
Accordingly, it would be desirable to provide an automated, fast-response, robust tracking and control system for lateral belt positioning in a hostile linear CMP environment. It would further be desirable to provide an automated, fast-response, versatile belt control and steering system for a hostile linear CMP environment, that selectively applies polishing pad regions having differing properties to processing selective areas of the wafer surface in order to achieve desired removal rates.